The present invention relates to a process for molding plastic articles by means of molds formed by joining two half-molds, this process being particularly suitable for producing articles which possess a reentrant or concave part situated on either side of the plane of separation of the half-molds. The invention also relates to molds which are particularly suitable for carrying out this process.
Plastics are being used to an increasing extent for the manufacture of a multiplicity of hollow articles such as drinking cups, bottles, drums, barrels, storage tanks and other containers.
In order to manufacture these products, the technique of blow-molding is generally employed, although other techniques such as injection molding, or cold or hot shaping, can, in some cases, also prove to be very effective.
In order to mold these articles, molds are generally used which are formed by joining two half-molds, and, after molding and cooling, the half-molds are separated from one another in order to make it possible to release the resulting article from the mold.
Some of these hollow articles, and this is true, for example, for many types of bottles, must, for reasons of mechanical strength, possess reentrant parts and, in particular, reentrant bases, that is to say bases which are concave towards the inside of the hollow article. These reentrant parts are frequently situated, as in the case of reentrant bases, on either side of the plane corresponding to the plane of separation of the half-molds in the axis of the mold.
If conventional molds are used to manufacture this type of article, it is found that when the half-mold or half-molds shift as the mold is being opened, they damage these reentrant parts and in some cases even tear them.
In order to make it possible to manufacture hollow articles possessing reentrant bases with a deep hollow, it has consequently been proposed to make use of improved molds composed of base parts which can be separated from the parts forming the body, and mechanisms have been developed for controlling the axial shift of the base parts prior to opening the body parts in order to make it possible to release the reentrant parts completely.
These molds and their control mechanism, however, require a substantial increase in the cost of the molding installations and make the latter more delicate. Moreover, these means make it necessary for a free space to exist underneath the base of the molds so as to permit the axial release of the parts forming the bases of the molds. This restriction makes it impossible to manufacture these articles in rotary machines equipped with molds which are practically touching and which are mounted on the periphery of a rotating plate or some other rotating support, and thus to benefit from the high productivity inherent in installations of this type.
It would consequently appear that it has hitherto not been possible to satisfactorily manufacture hollow articles and other articles possessing reentrant parts as defined above.